Bi- and triflavonoids represent a diverse family of dimers, trimers, and oligomers of flavonoid monomers that are not linked via the C-4 heterocyclic carbon and are consequently not classified as proanthocyanidins. They, unlike the proanthocyanidins, do not form coloured anthocyanidins upon treatment with acids. The flavonoids characteristically have a carbonyl group or equivalent at the C-4 position (Figure 1). Together with proanthocyanidins, the bi- and triflavonoids constitute the two major classes of “complex C6-C3-C6 secondary metabolites” (Ferreira; D; Slade, D; Marais, J. P. J. Flavonoids Chemistry, Biochemistry and Applications, edited by Øyvind M. Andersen Kenneth R. Markhams, CRC press 2006, 553-615, 1102-1135).

Bi- and triflavonoids that occur in nature are thought to be the products of phenol oxidative coupling of chalcones, aurones, auronols, isoflavones, flavanones, dihydroflavanols, flavanols and flavones. This contrasts with natural proanthocyanidins that are thought to be products of nucleophilic substitution of an hydroxy leaving group at C-4 of flavan-3,4-diols (via a C-4 carbocationic intermediate or via an SN2 reaction). The oxidative nature of carbon-carbon bond formation leads to preservation of the substituents at C-4 of the starting materials. Tetra-, penta-, and hexaflavonoids have also been isolated and identified, and these compounds are also included in the class of bi- and triflavonoids. The Locksley system of nomenclature for bi- and triflavonoids has been applied in the present specification. (Locksley, H. D. Fortschr. Chem. Org. Naturst. 1973, 30, 207 and Rahman, M.; Riaz, M.; Desai, U. R. Chem. & Biodiv. 2007, 4, 2495-2527; and refs therein) However, Locksley's suggestion that biflavonoids be referred to as biflavanoids has not been followed herein.
Compounds with a carbonyl group at C-4 that do not arise from phenol oxidative coupling of monomeric flavonoids, including compounds linked via carbon-oxygen bonds, have been reported as bi- or triflavonoids.
A growing number of “mixed” dimers e.g. flavan-3-ol→flavonol {e.g. fisetinidol-[4α→2′]-myricetin (6)}, originating from oxidative coupling of flavan-3-ols to flavonoids, has been isolated and belongs to both classes.

The structural diversity of bi- and triflavonoids makes the development of general synthetic methods impossible.
For simplicity, bi- and triflavonoids have, in terms of this specification, been categorised into four classes, depending on the nature and position of the interflavonoid bond:
Class 1: Bi- and triflavonoids where the carbon-carbon link is between aromatic rings. A plethora of synthetic methods exists to construct carbon-carbon inter-aromatic bonds (including Suzuki and Stille coupling reactions). These can be applied directly to bi- and triflavonoid synthesis (Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457; Muller, D.; Fleury, J.-P. Tetrahedron lett. 1991, 32 (20), 2229-2232; and Stille, J. K. Angew. Chem., Intl. Ed. Engl. 1986, 25, 508).
Class 2: Bi- and triflavonoids where the carbon-carbon link between the aromatic rings are replaced with a carbon-oxygen-carbon bond. Many synthetic methods exist to construct these inter-aromatic bonds (including the Ullmann coupling). These can be applied directly to bi- and triflavonoid synthesis (Nakazawa, K.; Ito, M. Chem. Pharm. Bull. 1963, 283; Ahmed, S.; Razaq, S. Tetrahedron 1976, 32, 503; and Zhang, F.-J.; Lin, G.-Q.; Huang, Q.-C. J. Org. Chem. 1995, 60, 6427).
Class 3: Bi- and triflavonoids where the carbon-carbon link is between the aromatic ring of one constituent monomer and the heterocyclic C-ring of another constituent monomer. As coupling of an aromatic ring on the 4-position of one constituent monomer will result in a proanthocyanidin, it leaves only C-2 and C-3 to couple. (I-3 and I-2 biflavones, including GB-flavones [I-3, II-8]-coupling and I-2 and I-3 bi-isoflavones). No I-2 biflavones have ever been isolated, unless one considers A-type proanthocyanidins as biflavonoids. Despite the interest that these I-3 flavones had received and their considerable pharmaceutical potential, no methods exists to synthesize any examples from this class (Ding, Y.; Li, X.-C.; Ferreira, D. J. Org. Chem. 2007, 72 (24), 9010-9017; Rucksthl, M.; Beretz, A.; Anton, R.; Landry, Y. Biochem. Pharmacol. 1979, 28, 535; Iwu, M. M.; Igboko, O. A.; Okunji, C. O.; Tempesta, M. S. J. Pharm. Pharmacol. 1990, 42, 290; Amelia, M.; Bronne, C.; Briancon, F.; Hagg, M.; Anton, R.; Landry, Y. Planta Med. 1985, 51, 16; Sun, C.-M.; Syu, W.-J.; Huang, Y.-T.; Chen, C.-C.; Ou, J.-C. J. Nat. Prod. 1997, 60, 382; Lin, Y.-M.; Flavin, M. T.; Cassidy, C. S.; Mar, A.; Chen, F. C. Bioorg. Med. Chem. Lett., 2001, 11, 2101; Chang, H. W.; Baek, S. H.; Chung, K. W.; Son, K. H.; Kim, H. P.; Kang, S. S. Biochem. Biophys. Res. Commun. 1994, 205, 843).
Class 4: Bi- and triflavonoids where the carbon-carbon link is between the heterocyclic C-rings.
Bi- and triflavonoids formed in accordance with class 3 have been proven to have biological activity in a variety of screens. Progress in drug development is, however, hampered because no stereoselective synthetic access exists to obtain larger quantities of pure compounds and all research is based on molecules isolated in small quantities from natural sources.
Optically active compounds are highly desirable and are characterized in that the compounds include at least one asymmetric carbon and that the resultant stereoisomers or enantiomers are not present in 50:50 racemic mixture but display an enantiomeric abundance or excess of one of the enantiomers. Such excess or abundance results in the property of the compound to cause rotation of the orientation of polarized light passed through a solution thereof, hence for its so-called optical activity. In the most preferred form an optically active compound would have only one enantiomer present in a composition thereof, and such compositions are thus referred to as being enantimerically pure. The Applicant is not aware of any literature describing synthetic methods to obtain examples of class 3 bi- and triflavonoids in enantiomerically pure or optically active forms or from flavan-3-ols, despite the fact that morelloflavone (4) exhibits anti-inflammatory activity. This invention is accordingly in particular concerned with the preparation of optically active biflavanoid compounds and biflavanoid analogue compounds but is also applicable to the preparation of optically inactive compounds which are produced from optically inactive starting materials as will be described in more detail below.
Ferreira and co-workers speculated about the intermediacy of a quinomethane radical (38) or (39) in the biosynthesis of naturally occurring 2,7-bibenzofuranoids (40) from maesopsin (41) or an α-hydroxychalcone (42) (Scheme 1) [Bekker, R.; Ferreira, D.; Swart, K. J.; Brandt, E. V. Tetrahedron 2000, 56, 5297-5302].

It has been submitted that the synthesis of the first natural [I-4, II-3″] bi-isoflavonoid (43), isolated from Dalbergia nitidula and synthesized by Roux and co-workers (Brandt, E. V.; Bezuidenhoudt, B. C. B.; Roux, D. G. J. Chem. Soc., Chem. Commun. 1982, 1408-1410 and Bezuidenhoudt, B. C. B.; Brandt, E. V.; Roux, D. G. J. Chem. Soc., Perkin Trans. I 1984, 2767-2778) via nucleophilic attack on an isoflavanyl-4-carbocation, generated from a pterocarpan (44), is a special case of the C-4 arylsubstituted proanthocyanidin syntheses. The B-ring of the isoflavanyl nucleophile is more oxygenated and reactive than its A-ring and a C4→C3′-linkage is formed (Scheme 2).

Donnelly and co-workers (Donnelly, D. M. X.; Fitzpatrick, B. M.; Ryan, S. M.; Finet, J.-P. J. Chem. Soc., Perkin Trans. I 1994, 1794-1801) synthesized the biflavonoids (46) and (47) via arylation of a 3-phenylsulfanylflavanone (48) with an 8-triacetoxyplumbylflavan derivative (49) (2:1 mixture of cis and trans). The dioxolane ring, of the intermediate (50) was cleaved during acid workup. Desulfurization of the intermediate (50) with nickel boride (NaBH4/H2O, NiCl26H2O/EtOH) yielded a chalcone (51) (in a 1.1:1 ratio of E/Z) that was cyclised with anhydrous sodium acetate in refluxing ethanol to the 3,8-biflavanone (46) in 73% yield (only the 2,3-trans isomer was isolated from a mixture of diastereoisomers with an α/β ratio of 1.4:1). Oxidation of (46) with dimethyldioxirane in acetone yielded (47) in 47% yield (Scheme 3). The end products are however not optically active and constitute a racemic mixture.

Synthesis of the isoflavone-isoflavone dimer (52), isolated from Dalbergia nitidula posed a challenge. It is linked via an electron deficient β-carbon in an α,β-unsaturated carbonyl moiety and no simple biomimetic equivalent of the proanthocyanidin syntheses was available. Ferreira and co-workers tried a variety of options, including coupling of nucleophilic phenolic units to aromatic oxygenated isoflavone-2,3-epoxides, direct coupling of phenolic units to isoflavones in a 1,4-Michael fashion, linkage of phenolic units to the acetal-type electrophilic centre of the intermediate in isoflavone synthesis prior to hetereocycle construction (thallium(III)nitrate strategy) (Farkas, L.; Gottsegen, A.; Nogradi, M.; Antus, S. J. Chem. Soc., Perkin Trans. I 1974, 305 and Antus, S.; Farkas, L.; Gottsegen, A.; Kardos-Balogh, Z.; Nogradi, M. Chem. Ber. 1976) and condensation of a C16 (5′-formylated isoflavan) (53) with a C14-unit (2-hydroxydeoxybenzoin) (54) in a modified Baker-Venkataraman reaction (Wagner, H.; Farkas, L. In: ‘The Flavonoids’, ed. J. B. Harbome, T. J. Mabry, and H. Mabry, Chapman and Hall, London, 1975, 138). The last strategy succeeded in producing the target dimer (52) (Scheme 4). The biflavanoid moiety of these end products are also not optically active.

The carboxylic moiety in (53) (the C16 unit) was introduced via formylation of the B-ring of (57) with the photochemical Reimer-Tieman reaction (Hirao, K.; Yonnemitsu, O. J. Chem. Soc., Chem. Commun. 1972, 812 and Hirao, K.; Ikegame, M.; Yonnemitsu, O. Tetrahedron 1974, 30, 2301). The deoxybenzoin (54) (the C14 unit) was obtained from oxidation of the chalcone (55) with Tl(NO3)3 to (56) followed by decarbonylation with perchloric acid.
In light of the above, it is evident that methods which achieve replacing the 3-hydroxy group of flavan-3-ols with a carbon-carbon bond and retains the optical activity of the starting material remain an elusive goal. This would open the way to a plethora of new classes of flavonoids, including naturally occurring 3-coupled biflavonoids (I-3, II-6/8 biflavonoids).
In this specification the expression “flavonoid compounds” is used to denote compounds which are based on the flavonoid base structure represented by the general formula F
and in which carbons C-2′ to C-6′ and C-5 to C-8 may be unsubstituted or may independently be substituted by —OH, hydrocarbyl moieties, saccharide moieties and —OR10; wherein R10 is selected from the group consisting of hydrocarbyl moieties, acyl moeities and benzyl moieties, and wherein the hydrocarbyl moeities and the acyl moieties each contains from 1 to 10 carbon atoms,as well as such unsubstituted or substituted compounds in which the C-3 and C-4 carbons may have double bond between them to constitute a flavene as herein defined,and also such unsubstituted or substituted compounds in which the C-2 and C-3 carbon atoms are bonded by a double bond and the C-4 carbon together with an oxygen atom bonded thereto may be a carbonyl moiety to constitute a C-4 flavone,and also such unsubstituted or substituted compounds in which the C-2 and C-3 carbon atoms are bonded by a single bond and the C-3 or the C-4 carbons together with an oxygen atom bonded thereto may be a carbonyl moiety to constitute a C-3 or C-4 flavanone,and also such unsubstituted or substituted compounds in which the C-3 or the C-4 may have a hydroxy moiety bonded thereto so that the compound constitutes a C-3 or C-4 flavanol.
For purposes of this specification, the term “flavene” thus denotes a flavonoid compound of the general Formula (A):
wherein the heterocyclic ring C of said flavonoid compound comprises a double bond between C-3 and C-4, and wherein the aromatic A- and B-rings may have a variety of different hydroxylation patterns and/or substituents, in particular H or OH, on one or more of the carbon atoms of said A- and B-rings.
The term “C-3 coupled biflavonoid” denotes a compound which is essentially a multimer (i.e. a dimer, trimer, tetramer etc.) of at least two monomeric units having flavonoid base structures and which are coupled together through a class 3 interflavanyl bond via the C-3 heterocyclic carbon.
The expression “C-3 coupled biflavonoid analogue” as used herein denotes a compound which is composed of at least two monomeric units, such that the first monomeric unit has a flavonoid base structure and the second monomeric unit has a non-flavonoid base structure, provided that said unit having a non-flavonoid base structure includes a nucleophilic aromatic moiety, and wherein the unit having a flavonoid base structure and the unit having a non-flavonoid base structure are coupled together through a class 3 interflavanyl bond via the C-3 heterocyclic carbon.